Polymer electrolyte fuel cell is generally a type of fuel cell utilizing ion exchange membranes as electrolytes. It has a compact structure with high power density, and can be operated with a simple system. Therefore, it is noted for portable or space ship electric power source as well as for a fixed distributed electric power source. In a prior art polymer electrolyte fuel cell, a fuel cell unit 7 has a membrane electrode assembly 3 including an ion exchange membrane 1 and gas diffusion electrodes 2a and 2b on both sides, reactant gas supply separators 5 for supplying fuel gas (or a first reactant gas) and oxidant gas (or a second reactant gas) to the gas diffusion electrodes 2a and 2b, respectively, and gaskets 6, arranged on the fringe portion of the separators 5 as shown in FIG. 1 which is an exploded cross-sectional view. The fuel cell is formed in a fuel cell stack 10 of a plurality of the fuel cell units 7 laminated as shown in FIG. 2 which is a cross-sectional view.
The ion exchange membrane 1 is typically a proton exchange membrane of par-fluorocarbon-sulfonic acid. The gas diffusion electrodes 2a and 2b are typically porous carbon plates with catalyzer layers such as platinum layers. One of the gas diffusion electrodes, a fuel electrode (anode) 2a, is supplied with fuel gas such as hydrogen, and the other electrode, an oxidizer electrode (cathode) 2b, is supplied with oxidant gas such as air. When the fuel electrode 2a is supplied with fuel gas including hydrogen as a main content and the oxidizer electrode 2b is supplied with oxidant gas such as air, electro-chemical reaction occurs at the pair of the electrodes 2a and 2b of the membrane electrode assembly 3 as shown in Equations 1 and 2, and electromotive force of about 1 V is generated:
At the fuel electrode:2H2→4H++4e−  (Eq. 1)
At the oxidizer electrode:O2+4H++4e−→H2O  (Eq. 2)
At the catalyzer layer of the fuel electrode 2a, the supplied hydrogen is dissociated into hydrogen ions and electrons as shown in Equation 1. The hydrogen ions move to the oxidizer electrode 2b through the ion exchange membrane 1, while the electrons move to the oxidizer electrode 2b through an external circuit. At the same time, at the catalyzer layer of the oxidizer electrode 2b, the oxygen in the supplied oxidant gas reacts with the hydrogen ions and the electrons described above, and water is generated, as shown in Equation 2. Then the electrons flowing through the external circuit generate electric current, and electric power can be supplied. The water generated in the reaction shown in Equations 1 and 2 is drained with the gas which is not consumed in the fuel cell (or reacted gas).
The ion exchange membrane 1 also has a function of preventing the reactant gases which are supplied to the fuel electrode 2a and the oxidizer electrode 2b, from mixing with each other, so that the area of the ion exchange membrane 1 is typically larger than those of the electrodes 2a and 2b. Through-holes called manifolds 8a and 8b are formed in the membrane electrode assemblies 3 and the reactant gas supply separators 5 such that the reactant gas can flow in the direction of the stacking of the fuel cell units 7. The characters “8a” denote the reactant gas supply manifold for supplying the reactant gas to each of the fuel cell units, while the characters “8b” denote the reactant gas exhaust manifold for exhausting the reacted gas from each of the fuel cell units.
In order to extract electric current from the membrane electrode assemblies 3, the reactant gases or the fuel gas and oxidant gas must be supplied to the electrodes 2a and 2b, respectively. At the same time, there must be elements for collecting electric charge adjacent to the electrodes 2a and 2b. Therefore, the reactant gas supply separators 5 arranged adjacent to the membrane electrode assemblies 3 have the function of supplying the reactant gases to the electrodes 2a and 2b, and also have the function of collecting electric charge.
As for supplying reactant gases to the electrodes 2a and 2b, fuel gas supply grooves 9a are formed on the surface of the reactant gas supply separator 5 facing the fuel electrode 2a, and oxidant gas supply grooves 9b are formed on the surface of the reactant gas supply separator 5 facing the oxidizer electrode 2b. The reactant gas supply manifold 8a is communicated to one end of each of the supply grooves 9a and 9b, and the reactant gas exhaust manifold 8b is communicated to the other end of each of the supply grooves 9a and 9b. 
When electric power is generated in the fuel cell stack 10, reaction heat is generated in each of the fuel cell units 7 in the reaction of Equations 1 and 2. Since the amount of the heat generated in the fuel cell stack 10 with a plurality of the fuel cell units 7 is large, cooling means is necessary for stable and continuous power generation operation. The conventional fuel cell stack 10 is cooled by coolant such as pure water or antifreezing fluid flowing through cooling plates which are inserted between the fuel cell units.
In recent years, so-called latent heat cooling is studied where the fuel cell units 7 are cooled by guiding mixed fluid of fuel gas and water in the fuel gas supply grooves 9a on the reactant gas supply separator 5 and by evaporating water. The water to be evaporated includes the water which has been moved from the fuel electrodes 2a to the oxidizer electrodes 2b through the ion exchange membrane 1 and the water which has been generated in the cell reaction in the oxidizer electrodes 2b. In the fuel cell stack 10 with latent heat cooling, electric power is generated by supplying mixed fluid of fuel gas and water to the fuel electrodes 2a, and air with unsaturated steam to the oxidizer electrodes 2b, wherein the amount of water in the fuel gas is more than the amount of water evaporated on the oxidizer electrodes 2b. 
On the oxidizer electrodes 2b, not only the water generated by the electric power generation but also the water moved from the fuel electrodes 2a to the oxidizer electrodes 2b through the high polymer membrane 1 evaporate. The water evaporating at the oxidizer electrode 2b can absorb a large amount of latent heat, and the fuel cell units 7 can be cooled. By the latent heat cooling, the cooling plates can be eliminated in the fuel cell stack 10, and a large amount of cooling water circulation is not needed. Therefore, the fuel cell stack 10 itself and the power generation system utilizing the stack 10 can be designed in compact and light-weight structure, while cooling of the stack is secured.
In the fuel cell stack 10 having plurality of the fuel cell units 7, it is important that the reactant gas supply separators 5 supply reactant gases uniformly to each of the electrodes 2a and 2b. Equal distribution of fuel gas and water is especially important in the fuel cell stack 10 using latent heat cooling, because the fuel gas supply grooves 9a on the reactant gas supply separator 5 have to supply mixed fluid of fuel gas and water to the fuel electrode 2a. If the fuel gas and water distribution to the fuel electrodes 2a become non-uniform, the cell performance of the fuel cell unit 7 including the fuel electrode 2a may deteriorate.
Specifically, if there are any fuel cell units 7 which are not supplied with sufficient fuel gas because of non-uniform distribution of the fuel gas, the cell performance deteriorates and even electrolytic corrosion may occur on the fuel electrodes 2a and power generation may not be continued. If there are any fuel cell units 7 which are not supplied with sufficient water because of non-uniform distribution of water, the latent heat cooling capability may deteriorate, and the fuel cell units 7 may have insufficient cooling, then their temperature may rise. As a result, the cell performance may deteriorate, and power generation may not be continued.
Non-uniform distributions of the fuel gas and water depend upon the methods of supplying the water to the fuel gas. In the prior art, there have been disclosed some water supplying methods. However, none of them have disclosed countermeasures for the nonuniformity of distribution to the fuel cell units 7 of the fuel cell stack 10. For example, in Japanese Patent Application Publication Hei 1-140562, spray water is supplied to the fuel electrodes through a spray sparger with aspirator effect. However, the sprayed water would be collected together into water drops to form two-phase flows. Therefore, it would be difficult to distribute the mixed fluid of the fuel gas and water into the fuel cell units. It would be difficult especially during high loading electric current operation when large amount of heat is generated.
Japanese Patent Publication Hei 7-95447 discloses a method of cooling a fuel stack by evaporation latent heat, wherein porous plates are placed on the fuel electrodes, water is supplied from the fuel electrodes to the ion exchange membrane by pressing water into the plates, and excessive water is evaporated into fuel gas. However, in this technique, the pressure difference between the fuel gas and the supplied water must be finely controlled. Therefore, it is difficult to adjust the amount of supplied water, and it is difficult to distribute water uniformly to the fuel cell units.
Japanese Patent Application Publication Hei 5-41230 discloses a method of supplying water to the ion exchange membranes by supplying water to ribs. The ribs are formed on the fuel electrodes (ribbed electrodes) and have grooves for fuel gas to pass through. According to this method, relatively equal water distribution to the fuel cell units is obtained, because wicking phenomenon of water into the air pores in the ribbed electrodes is utilized. However, if sufficient amount of water were supplied to the ribs for latent heat cooling of the heat generated in the whole fuel cell stack, most of the air pores in the ribbed electrodes would be filled with water. The fuel gas must be diffused in the catalyst layers of the fuel electrodes through the air pores in the ribbed electrodes, and the fuel gas diffusion is obstructed if the air pores are filled with water, which is called a “flooding” phenomenon. Thus, the cell voltage would decrease. Therefore, if the amount of supplied water is increased, stable operation of the fuel cell stack will be difficult, and then, this method may not be applied to latent heat cooling of the fuel cell stack.
Japanese Patent Application Publication Hei 7-220746 discloses a method of supplying water to the reactant gas by providing the separators with reactant gas supply grooves, with a header and a pipe for supplying water to add moisture. According to this method, relatively equal water distribution is obtained in each of the separators, because the water is supplied directly to the gas supply grooves. However, this reference does not disclose the technique how to distribute uniformly in the fuel cell units in a fuel cell stack having many fuel cell units.
Furthermore, high positioning preciseness is required to provide the manifold with the header and the pipe for supplying water to add moisture, and to connect the pipe to the reactant gas supply grooves. Thus, this method results in high cost in production. In addition, since the fuel cell stack is constructed by stacking many membrane electrode assemblies, reactant gas supply separators, etc., inadequate preciseness in positioning in stacking would adversely and heavily effect the reactant gas and water distribution. Therefore, high positioning preciseness is required in stacking, and the working cost for stacking is high. Such high working cost is a general problem in all fuel cell stacks including those discussed above.
Furthermore, according to the method discussed above, even if equal flow distribution is possible when power is generated with a fixed fuel cell stack, the water flow distribution to the fuel gas becomes non-uniform when the fuel cell stack tilts or vibrates. Then, fuel cell units may have non-uniform distribution in latent heat cooling capability, and the cell performance of each fuel cell unit may deteriorate. Especially when the fuel cells are used for vehicles, tilt and vibration of the fuel cell stacks are unavoidable, stable power generation would be difficult.
Furthermore, if bubbles enter into the water which is supplied to the fuel gas, the water distribution would become non-uniform, and the cell performance of the fuel cell units would deteriorate. Therefore, it is important to vent gas from the water which is to be supplied to the fuel gas. Thus, a gas vent device is indispensable, and simplified system has been demanded. By the way, gas vent devices are generally used not only for fuel cell stacks, and their improvement has been always demanded.